Electrochemical cell with at least one gas diffusion electrode

ABSTRACT

An electrochemical cell has at least one positive wet-proofed gas diffusion electrode with its major portion within the casing covered with a fuel and oxidant impervious mask, and with its remaining portion within the casing covered with at least one water soluble polymeric thickening agent swellable in the aqueous alkaline electrolyte of a fuel-electrolyte solution and an alkali metal silicate dissolved in the solution. This cell eliminates or reduces substantially the conventional problems of carbonate crust formation, electrolyte leakage, drowning of the gas diffusion electrode, and loss of fuel.

United States Patent Grubb et al. 51 Mar. 21, 1972 [54] ELECTROCHEMICALCELL WITH AT 3,244,564 4/]966 Fox l 36/86 LEAST ONE GAS DIFFUSION IELECTRODE Primary Examiner-Wmston A. Douglas Assistant Examiner-H. A.Feeley [72] Inventors: Willard T. Grubb, S h e t d N Y Attorney-RichardR. Brainard, Paul A. Frank, Charles T. Robert Mac", Milwaukee, w Watts,Paul R. Webb, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman[7 3] Assignee: General Electric Company 221 Filed: Apr. 6, 1970 [571ABSTRACT An electrochemical cell has at least one positive wet proofedl. 5 [211 App No 2 gas diffusion electrode with its major portion withinthe casing covered with a fuel and oxidant impervious mask, and with its[52] US. Cl. ..136/86 R remaining port on within the casing co ered withat least one [51] I t, Cl. ]{01 27/00 water soluble polymeric thickeningagent swellable in the 58 Field of Search ..l36/86, 157, 14s aqueousalkaline electrolyte of a fueleleetrelyte Solution and an alkali metalsilicate dissolved in the solution. This cell 56] References Citedeliminates or reduces substantially the conventional problems ofcarbonate crust formation, electrolyte leakage, drowning of UNITEDSTATES PATENTS the gas diffusion electrode, and loss of fuel.

1,541,699 6/1925 Freeth et al ..l36/l57 5 Claims, 1 Drawing FigurePatented March 21, 1972 3,650,83

//V VE/V TOPS WILLARD r muss, ROBERT A. MACUR by FM XVVWIL Thisinvention relates to electrochemical cells with at least one gasdiffusion electrode and, more particularly, to electrochemical cellswith a positive wet-proofed and coated and a fuel-electrolyte solutionadditive.

An electrochemical cell with at least one gas diffusionelectrode whenemployed as a fuel cell having a solution of an aqueous alkalineelectrolyte with a soluble fuel dissolved therein, contains an anode incontact with the electrolyte that is capable of electrocatalyticallyoxidizing a fuel, such as an alcohol. The cathode is a porous, gasdiffusion electrode which forms a barrier between the electrolyte andthe oxidant, usually air. The cathode is provided with a surface whichis active for the reduction of the oxidant but is catalytically passiveto the alcohol or other fuel. In such cells, serious problems areencountered in leakage of the electrolyte from the casing, drowning ofthe cathode by the electrolyte, and evaporation of the fuel.

The gas diffusion electrode in'the above type of cell may'act as a wickfor the electrolyte. Upon the cell sstanding in air for several days,formation of carbonate occurs in the electrolytic solution and carbonatecrust may be formed on the exterior surface of the gas diffusionelectrode. This crust reduces the cell performance and may lead tophysical degradation of the electrode structure. As the carbonate crustforms on the exterior surface of the gas diffusion electrode the wickingaction of the electrode does not stop at the physical limits of theelectrode but the crust wicks up more solution and expands further thevolume of the crust formation. Subsequently, all of the solution fromthe cell is eventually removed down to the level of the lower end of theelectrode by this wicking action.

Attempts have been made to overcome this serious problem of carbonatecrust formation by wet-proofing the gas diffusion electrode. When thishas been accomplished the electrolyte penetrates only a limited distanceinto a porous structure. Carbonate crust formation will still occur onthe exterior surface of the electrode but to a lesser amount. However, asecond problem occurs in that carbonate crust forms in the pores of thestructure into which it has penetrated whereupon its expansion causescracking of the electrode.

In the copending patent application of Willard T. Grubb and Carl E.Cliche, Ser. No. 725,989, filed May 2, 1968 and entitled CompositeElectrode and Electrochemical Cell With At Least One Gas DiffusionElectrode, there is described and claimed a composite article, anelectrochemical cell and a method of forming a composite article. Theelectrochemical cell of this copending application provides for aportion of the gas diffusion electrode to have thereon a layer of atleast one water soluble polymeric thickening agent swellable in anaqueous electrolyte. This copending application is assigned to the sameassignee as the present application.

In US. Pat. No. 3,4 l 8,l66 issued Dec. 24, 1968, an alkaline storagecell is described which has an alkali metal silicate dissolved in theelectrolyte, incorporated in the cadmium or iron negative material, orincorporated in the positive active material to extend the life of thenegative active material.

It is a primary object of our invention to provide an improvedelectrochemical cell which eliminates or substantially reduces carbonatecrust formation on the exterior surface of the gas diffusion electrode.

In accordance with one aspect ofour invention, an electrochemical cellincludes the improvement of a major portion of the positive electrodewithin the casing being covered with a fuel and oxidant impervious mask,the remaining portion of the positive electrode within the casingforming an interface with the electrolyte, the remaining portion of thepositive electrode being covered with at least one water solublepolymeric thickening agent swellable in the aqueous alkalineelectrolyte, and an alkali metal silicate dissolved in thefuel-electrolyte solution.

These and various other objects, features, and advantages of theinvention will be better understood from the following description takenin connection with the accompanying drawing in which:

The single FIGURE is a vertical, sectional view of a fuel cell embodyingour invention.

In the single FIGURE of the invention, there is shown generally at 10 anelectrochemical cell with at least one gas diffusion electrode embodyingour invention, which cell is shown as a fuel cell. Cell 10 has a casing11 which comprises a body portion 12 with a closed bottom and an opentop and a cover portion 13 which fits tightly over the open top of bodyportion 12. Casing ll defines a chamber 14 in which there is providedfor the operation of the cell a solution of alkaline electrolyte such aspotassium hydroxide and a dissolved fuel such as methanol. An alkalimetal silicate such as sodium metasilicate or potassium metasilicate isadded to the electrolytic solution. A wet-proofed positive porous gasdiffusion electrode or cathode 15 has one end fitted into an aperture 16in cover 13 of casing 11. In this manner, the upper surface of thecathode is exposed to the oxygen oxidant of the atmosphere exterior ofcasing 11 while the remaining portion of the cathode extends downwardlyinto chamber 14.

An anode 17 is positioned close to but spaced from cathode 1 5 withinchamber 14. While various suitable arrangements can be employed tosupport anode 17, there is shown an electrical lead 18 for such supportwhich lead is affixed to anode l7 and extends upwardly through cover 13of casing 11. Similarly, a lead 19 is shown in contact with cathode 15and extends outwardly through cover 13 of casing 11. A fuel and oxidantimpervious mask 20 covers or overlies a major portion of porous cathode15 except for a portion 21 of its surface which faces or is opposed toanode 17. Portion 21 forms an interface with the electrolytic solution.At least one water soluble polymer gelling agent 22 is shown coveringportion 21 of cathode 15. The same agent 22 is also shown covering theexterior surface of anode 17, and the interior of both body portion 12and cover 13 of casing 11.

We found that the combination of wet'proofing, masking, thickening agentand alkali metal silicate resulted in an improved electrochemical cellin which carbonate crust was eliminated or reduced substantially. If adefect forms in the thickening agent layer on the gas diffusionelectrode, penetration of the carbonate will normally occur into thepores of the wet-proofed electrode. However, the alkali metal silicatereduces this penetration and prevents any carbonate crust from formingin the pores of the electrode or on the exterior surface of theelectrode.

The preferred electrolytic solution of electrolyte and fuel is a mixturewhich is confined or stored in chamber 14 of an aqueous alkalineelectrolyte and an alcohol. As is well understood in the art, theelectrolyte may be an aqueous alkali solution of any desiredconcentration. While the alkali metal hydroxides are the most commonlyemployed and of the lower cost, it is appreciated that other solublehydroxides may be employed, such as tetrasubstituted ammonium orphosphonium hydroxides. As used herein the term alcohol includesmethanol, ethanol, n-propanol, and isopropanol-that is, any alcoholhaving from I to 3 carbon atoms. Methanol is a preferred alcohol on thebasis of cost and reactivity. Since the alcohols are miscible with waterin all proportions, any desired quantity may be utilized. It ispreferred, however, that the relation of hydroxyl ions to alcohol bemaintained in a molar ratio that insures that all the alcohol will beconsumed during extended operation of the unit. Other fuels are alsosuitable such as hydrazine, formate, ammonia, ethylene glycol, etc. Ifdesired, a large storage chamber for the electrolyte and fuel can beprovided by enlarging chamber 14 beneath the electrodes. In this manner,the alcohol capacity of the cell is increasedwithout in any waypenalizing performance of the cell. The alkali metal silicate, which isadded to the fuel-electrolyte solution is preferably sodium'metasilicate or potassium metasilicate.

The anode electrode is schematically illustrated in the drawing. It maybe of any conventional construction or configuration. Anodes. capable ofefficiently oxidizing alcohol usually include platinum metals or alloysas the electrocatalysts. Alloys of noble metals have been found to bemost efficient. The electrocatalyst may be employed as a high areacoating on a substrate, suitably bonded into a porous mass or supportedon a porous substrate of nickel, carbon, etc.

The porous cathode electrode is formed of a porous electronicallyconductive mass which, at least at the point where the conductive massforms an interface with the electrolyte, includes anelectrocatalytically active material. The electrocatalyst iscatalytically inactive toward the oxidation of alcohol. A preferredsuitable material meeting all of the above criteria is a porous carbonmass. This material is recognized to selectively catalyze the reductionof oxygen while remaining passive toward alcohol. Other catalysts are,of course, known which are selective to the reduction of oxygen. Silverand silver-nickel alloys are such materials. In the preferred formwet-proofing material is distributed throughout the conductive mass ofthe cathode.

A major portion of the cathode is covered with a mask. The mask may beformed of any material which is impervious to oxidant as well as fueland electrolyte. In the configuration shown in the single FIGURE of thedrawing, the function of the mask is to avoid contact of oxidant withfuel and electrolyte on the surface of the conductive mass, except atthe intended electrolyte interface. Specific examples of maskingmaterials include impervious metal coatings as well as coatings ofalkali and methanol impervious nonmetals, such as waxes, rubbers, andresins. The mask is preferably applied in the form of a coating. Wax hasbeen found to be an easily applied, low cost masking material ofsuitable stability. Cathodes of the above configuration which are bothmasked and wetproofed perform better in our invention than correspondingelectrodes lacking either masking, wet-proofing, or both. Masking of theabove type of cathode is undertaken to slow alcohol evaporation from acell, as will be more fully explained below.

To replenish the oxygen consumed at the cathode-electrolyte interface,air must diffuse through the porous, electronically conductive cathode.In the configuration shown in the single FIGURE of the drawing,impervious mask 20 is provided to limit the interface of the electrolyteand porous conductive mass to the portion 21 facing the anode electrode.Thus, alcohol diffusing from the restricted interface to the upper endofthe porous cathodes is provided with a relatively long diffusion path.The rate of loss of alcohol from the cell over long time periods is thussignificantly curtailed.

The exposed or unmasked portion 21 of cathode 16 is impregnated andcovered with at least one water soluble polymeric thickening agentswellable in the aqueous alkaline electrolyte of the electrolyticsolution. Additionally, if desired at least a portion of the interiorsurfaces of both the body portion and the cover of the cell casing canbe covered with a layer of the thickening agent. Similarly, a layer ofthe thickening agent can be applied to at least a portion of one of thesurfaces of the anode electrode. The water soluble polymeric thickeningagent is mixed initially with a solvent such as water and stirred, ifdesired, to form a mucilage or a viscous solution. This solution is thenapplied in any suitable manner to the exposed portion of the cathodeelectrode, to the anode electrode, and to the interior of both the bodyportion and to the cover of the cell casing. Subsequently, the watersolvent is allowed to evaporate from the solution thereby forming alayer of a thickening agent impregnated into, if the structure isporous, and covering the portion of the contacted structure.

The polymeric thickening agent must be water soluble and swellable in anaqueous electrolyte, but not mobile in the same electrolyte. Thethickening agent must adhere to and cover the portion of the substrateon which it is applied as a layer or film.

Suitable thickening agents which have the above properties and can beemployed in the practice of this invention and are swellable in anaqueous alkaline electrolyte include the polymers of acrylic acid,carboxy polymethylene, carboxymethyl cellulose, methyl cellulose, algin,polyethylene oxide, polyvinyl alcohol, and mixtures thereof. Suitablethickening agents which can be employed in the practice of our inventionand are swellable in an aqueous acid electrolyte include the polymers ofacrylic acid, carboxy polymethylene, algin, pectin, polyvinyl alcohol,carboxymethyl cellulose, and mixtures thereof.

The alkali metal silicates can add to the fuel-electrolyte solution invarious proportion with the unexpected beneficial results of preventingcarbonate crust formation and eliminating any wicking action through theformed moist residue. 0f the alkali metal silicates we prefer to usesodium metasilicate or potassium metasilicate.

In an illustrative operation of the cell shown in the single FIGURE ofthe drawing, portion 21 of cathode 15, had applied thereto a thickeningagent formed by mixing together at least one water soluble polymericthickening agent swellable in an aqueous electrolyte and a watersolvent. Previously cathode 15 had been wet proofed and covered over amajor portion with an impervious mask. After the above portions of thecell have dried, an electrolyte of potassium hydroxide and methylalcohol forming an electrolytic solution is poured into chamber 14 ofcasing 11. Generally, this solution is added to a level beneath the opentop of body portion 13. Potassium metasilicate is added to thefuel-electrolyte solution. Cover 13 is fitted functionally at the upperopen end of body portion 12. Anode l7, and the interior surfaces of bodyportion 12 and cover portion 13 of casing 11 had applied thereto thesame thickening agent. The thickening agent swells in the electrolyteimmobilizing substantially the electrolyte and dissolved fuel. Leads 18and 19 are connected to a suitable electrical load (not shown) andelectrical energy is generated from the cell. When the fuel in the formof the methyl alcohol has become sufficiently depleted, cover 13 can beremoved, the thickening solution in chamber 14 removed, and additionalpotassium hydroxide, methyl alcohol, and potassium metasilicate added tochamber 14. The cell can be recovered and operated.

An example of an electrochemical cell, which was not made in accordancewith our invention, is set forth below in Example I while an example ofan electrochemical cell, which was made in accordance with ourinvention, is set forth below in Example II.

EXAMPLE I A porous carbon block with dimensions of k X 5% X 1 A inch waswet-proofed with 9 weight percent of polyethylene. The block wasincorporated into an electrochemical cell structure as in the singleFIGURE of the drawing. The cell was filled with an aqueous 6 molarmethanol, 13 normal potassium hydroxide solution. The cell was placed ona resistive load. After 600 hours the cell showed carbonate crustformation on the air-exposed surface of its cathode. After 1,400 hours,the cell showed massive carbonate crust formation and was producing only15 percent of its initial voltage under load.

EXAMPLE II A porous carbon block as in Example I was drilled out with ainch diameter drill to within about Vs inch of one end forming a longcup shaped piece. This was wet-proofed with 10.6 weight percentpolyethylene. After wet-proofing, the outer surface of the porous carbonwas masked with a mask impervious to fuel, electrolyte and oxygen exceptfor an area 1 inch X l inch later to face the anode in the structureshown in the single FIGURE of the drawing. The unmasked surface wascoated with a layer of gel by applying two successive coats of amucilage prepared by dissolving 2.5 percent of a carboxylatedpolymethylene polymer in water. The layer was dried between coats andafter the second coat. This cathode was incorporated into anelectrochemical cell structure similar to the single FIGURE of thedrawing and placed on resistive load.

After 600 hours this cell showed no crust formation and there had beenno leakage of electrolyte into the cup-shaped cathode. It was delivering97 percent of its initial voltage under load. After 1,500 hours, therewas no crust formation and the cell was producing 56 percent of itsinitial voltage under load.

While other modifications of the invention and variations thereof whichmay be employed within the scope of the invention have not beendescribed, the invention is intended to include such as may be embracedwithin the following claims:

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In an electrochemical cell comprising a casing, at least one negativeelectrode positioned in the casing, at least one positive gas diffusionelectrode being uniformly impregnated with a wet-proofing materialpositioned in the casing and spaced from the negative electrode, oneportion of the positive electrode being exposed to oxygen oxidantexterior of the casing, a solution of an aqueous alkaline electrolytewith alcohol as the fuel dissolved therein within the casing and incontact with the electrodes, the improvement comprising'a major portionof the positive electrode within the casing being covered with a fueland oxidant impervious mask, and the remaining portion of the positiveelectrode within the casing forming an interface with the electrolyte,the remaining portion of the positive electrode being covered with atleast one water soluble polymeric thickening agent swellable in theaqueous alkaline electrolyte, and an alkali metal silicate in thefuel-electrolyte solution thereby reducing alkali carbonate crustformation on the exterior surface of the gas diffusion electrode.

2. In an electrochemical cell as in claim I, wherein the alkali metalsilicate is potassium metasilicate.

3. In an electrochemical cell as in claim 1 wherein the alkali metalsilicate is sodium metasilicate.

4. In an electrochemical cell as in claim 1, wherein the gas diffusionelectrode is porous carbon, the thickening agent is a carboxypolymethylene polymer, the electrolyte is potassium hydroxide, the fuelis methanol, and the alkali metal silicate is potassium metasilicate.

5. In an electrochemical cell as in claim 1, wherein the gas diffusionelectrode is porous carbon, the thickening agent is carboxypolymethylene polymer, the electrolyte is sodium hydroxide, the fuel ismethanol, and the alkali metal silicate is sodium metasilicate.

2. In an electrochemical cell as in claim 1, wherein the alkali metal silicate is potaSsium metasilicate.
 3. In an electrochemical cell as in claim 1 wherein the alkali metal silicate is sodium metasilicate.
 4. In an electrochemical cell as in claim 1, wherein the gas diffusion electrode is porous carbon, the thickening agent is a carboxy polymethylene polymer, the electrolyte is potassium hydroxide, the fuel is methanol, and the alkali metal silicate is potassium metasilicate.
 5. In an electrochemical cell as in claim 1, wherein the gas diffusion electrode is porous carbon, the thickening agent is carboxy polymethylene polymer, the electrolyte is sodium hydroxide, the fuel is methanol, and the alkali metal silicate is sodium metasilicate. 